A rolling mill, particularly one intended for processing metal materials, comprises a cage within which are placed at least two cylinders, and often more than two, aligned vertically along a gripping plane. The product to be processed, for example a metal strip or bar, is constrained to pass between two working cylinders serving to reduce its thickness and connected for this purpose, either directly or via other cylinders called support cylinders, to one or more control members which determine their vertical displacement along the vertical uprights of the cage in order to adjust their spacing, when idle or under load, as a function of the thickness of the product being rolled, i.e., to effect appropriate gripping.
A rolling mill of the type called "quarto", for example, comprises two working cylinders bearing respectively on two support cylinders. In a rolling mill of the "sexto" types intermediate cylinders are interposed between the working cylinders and the support cylinders.
The cage of the rolling mill normally comprises two vertical uprights connected by a crosspiece on which the gripping means bear. Each cylinder is supported by a shaft rotating, at both of its ends, in bearings mounted in a piece forming a bearing block, called a "chock", and able to slide, in the gripping plane, within the corresponding upright of the cage.
The gripping of the cylinders can be effected by mechanical devices of the type having a movable screw and fixed nut, in which the screw is moved by a toothed wheel of a reducing motor assembly of conventional type. However, it is generally preferred to use a hydraulic gripping device comprising at least two hydraulic jacks placed at the level of the two uprights of the cage and bearing on one side on the upright and on the other side on the chock of the corresponding cylinder, for example the upper support cylinder in the case of a quarto rolling mill.
Each jack comprises a plunger piston forming a hydraulic piston, and a body surrounding the piston and forming a shell defining a chamber of variable volume which is fed with fluid under pressure, the piston and body being able to slide axially one relative to one another. The body can be fixed and the piston movable, or vice versa, depending on the embodiment. In the first case, the movable piston bears, with its end opposite the pressure chamber, on the corresponding chock, and in the second case it is the movable body which bears directly on the chock.
A hydraulic gripping device possesses numerous advantages relative to the former screw devices. In particular, the hydraulic gripping device makes it possible to take more rapid action and to compensate virtually instantaneously the yielding of the cage, i.e., the movement apart of the rolling cylinders brought about by the engagement between these cylinders of the product to be rolled.
Furthermore, some time ago, very efficient systems were developed for adjusting the gripping and also compensating the various deformations, such systems requiring simple and accurate monitoring of the displacements of the movable element of the jack and of the chock. For this purpose, use is made of position sensors connected to hydraulic and electro-hydraulic elements forming the control circuit of the hydraulic gripping device.
In order conveniently to effect and monitor the measurements, it is logical to place the position sensor outside the cage, above the upper part of the corresponding upright. The sensor is then connected to the chock, whose position it monitors by means of a measuring rod, of constant length, which runs in a bore made within the upright or the crosspiece. In order to effect the measurement with accuracy, it is preferable for the rod to be placed in the axis of the jack and, as its end has to be connected to the movable part of the jack, it is caused to pass successively, via suitable apertures, through the upper part of the upright of the cage and the first and then the second member of the jack, before reaching the chock to which it is fixed by an attachment piece. As the rod passes through the chamber of the jack, it is necessary to ensure sufficient leak-tightness to resist the very high pressures existing in the jack, and for this purpose use is generally made of gaskets forming a sealing joint and placed at two levels in the apertures for the passage of the rod made on the two elements of the jack. In the most common embodiments such gaskets are arranged in the actual wall of the two elements of the jack and the rod is in permanent friction against these gaskets.
However, the measuring rod must accurately transmit to the position sensor the displacements, even minimal displacements, of the chock in question, and it must furthermore possess sufficient lateral flexibility for the measurement not to be falsified by the stresses exerted on the chock.
The arrangement adopted for the assembly of the measuring rod must therefore simultaneously meet a number of conditions in order for the measurement to be performed with accuracy.
The arrangements used to date apparently gave satisfaction, but it has recently been found that the requirements of accuracy in setting the position of the cylinders had become such that any disturbances, even very minimal disturbances such as, for example, the friction exerted by the sealing gasket, were capable of influencing the measurement.